Polyimides have long been recognized as materials exhibiting excellent thermal and thermo-oxidative stability, as well as providing superior mechanical and electrical properties, including good solvent resistance and light stability. These attributes are to a large measure responsible for the wide acceptance which such materials have received in the aerospace, electronics and other fields. As a consequence, polyimides have enjoyed widespread use, for example, in films, coatings, and molded products, particularly those intended for high temperature applications.
Such desirable properties have not, however, avoided certain less desirable characteristics of polyimides such as their tendency to decompose before melting and their general lack of solubility in ordinary solvents. Such short-comings have made it difficult, for instance, to process polyimide materials into fibers displaying the advantageous properties referred to.
In an attempt to overcome the described solubility and melting problems associated with the preparation of fibers, resort has often been had to the polymerization of the diamine monomers with tetracarboxylic dianhydrides in a suitable organic solvent at room temperature to yield a soluble intermediate polymer in the form of a polyamic acid. The reaction is readily accomplished in a number of suitable solvents, such as, for instance, N,N-dimethylformamide, DMF; N-methylpyrrolidine, NMP, and the like, from solutions from which the desired fibers can readily be spun. Following such step, the polyamic acid fiber is converted into the corresponding polyimide fiber by dehydration effected with heat or chemical dehydration agents.
While the two-step process detailed results in the production of the sought-after polyimide filamentary products, certain disadvantages encountered in the process are difficult to avoid. For example, the imidization reaction being reversible can lead to products having relatively low molecular weights that exhibit undesirable reactivity and mechanical weakness. Furthermore, the process often results in the creation of undesirable voids in the resulting filaments due to the water by-product formed during the reaction. High degrees of imidization are also difficult to obtain in fibers. An example of the two-step process set forth in the preceding is shown, for example, in Japanese Patent No. 1,260,015. The process there described involves the reaction of 2,2'-dimethyl-4,4'-diaminobiphenyl with pyromellitic dianhydride, PMDA, in a first step. After being spun, the fibers are imidized in a second step by thermal treatment.
Recently, however, it has been found that certain polyimides are sometimes soluble in particular solvents, and in such instances, polymers can be produced through use of a so-called one-step process in which the polyimides can be prepared in a single step. Thus, it has been found that 3,3',4,4'-biphenyltetracarboxylic dianhydride, BPDA, and pyromellitic dianhydride, PMDA, can be reacted with 3,3'-dimethyl-4,4'-diaminobiphenyl, OTOL, in a solution in which phenol, p-chlorophenol, m-cresol, p-cresol or 2,4-dichlorophenol are employed as reaction mixture solvents. The conversion to the polyimide takes place in solution, and fibers can be directly spun therefrom; "Hi-Strength-Hi-Modulus Polyimide Fibers I. One-Step Synthesis of Spinable Polyimides." Takaho Kaneda, et al.
Similarly, U.S. Pat. No. 5,071,997 teaches that segmented rigid-rod polyimides synthesized from BPDA and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, FMB, can be spun in a one-step process from hot m-cresol.
The fact that suitable solvents for such one-step methods are unpredictable, however, may be gathered from the fact that the patent referred to indicates that the polyimide there disclosed has but limited solubility in standard halogenated hydrocarbon solvents. Such unpredictability makes it difficult, therefore, to practice the one-step process other than with known systems.
One of the important reasons for the desirably high moduli of aromatic polyimides arises from the fact that the aromatic rings of the compounds are joined by rigid imide rings. Such structures provide the stiff backbones that favor formation of strong, rigid fibers. However, information relating to polyimide fibers is recognized by those familiar with the art as being limited, a lack which likely stems from the difficulty in finding materials suitable for solubilizing high molecular weight polyimides.
A further factor of significance in determining the physical properties of aromatic polyimides, however, is the presence or absence of substituent groups associated with the aromatic ring portions of the compounds. In this regard, both the nature and location of such substituents on the rings greatly influences the properties of the polyimide materials. As will be set forth more particularly in the following, the nature and position of such substituents appears to be important both to the manner of the polyimides' rates of crystallization, their degree of crystallinity, their crystalline packing as well as to their solubility in solvents, the latter being determinative of whether or not the polyimides are susceptible to fabrication by means of the preferred one-step process.
In view of the foregoing, therefore, it is a first aspect of this invention to provide polyimide fibers with improved physical properties.
A second aspect of this invention is to provide segmented, rigid-rod polyimide fibers that do not need to be spun from a polyamic acid solution.
An additional aspect of this invention is to provide polyimide polymers having specific substituent groups located in specific positions on the aromatic portions thereof.
A further aspect of this invention is to provide segmented, rigid-rod polyimide polymers which, together with the reactants required therefore, are soluble in a particular solvent.
Another aspect of this invention is to provide a reaction mixture for the synthesis of segmented, rigid-rod polyimide polymers from which polyimide fibers can be formed directly following formation of the polymers in solution.
Yet another aspect of this invention is to provide segmented, rigid-rod polymers with desirable crystallization and morphology characteristics.
A still further aspect of this invention is to provide segmented, rigid-rod fibers that exhibit crystallization rates which facilitate their drawing.